Quest for ion–ion correlations in electric double layers and overcharging phenomena

A discussion is given of the experimental evidence for overcharging, also known as charge reversal. The phenomenon is very common. The most general explanation is specific chemical adsorption. However, overcharging can also be explained on the basis of ion-ion correlations, which is rather a physical type of interpretation, and which can also be specific. Several theories have been developed for that. So, there is the luxury problem that two alternative explanations are available for the same observation. The purpose of this paper is to consider the various approaches critically and try to devise experimental options to discriminate between the two interpretations.